The kidnappers of a Tunisian diplomat in Tripoli have demanded the release of Islamist militants detained in Tunisia, Tunisia's foreign minister said on Friday.

The diplomat was kidnapped on Thursday in the Libyan capital, the second one within a month.

"We have been able to learn the identity of the kidnappers," Foreign Minister Mongi Hamdi said. "They belong to the family of a group of terrorists detained in Tunisia because of their involvement in attacks (three years ago) against security forces in Rouhia city."